diff mbox series

[v2,08/11] tpm: add session encryption protection to tpm2_get_random()

Message ID 20230124175516.5984-9-James.Bottomley@HansenPartnership.com (mailing list archive)
State New, archived
Headers show
Series add integrity and security to TPM2 transactions | expand

Commit Message

James Bottomley Jan. 24, 2023, 5:55 p.m. UTC
If some entity is snooping the TPM bus, they can see the random
numbers we're extracting from the TPM and do prediction attacks
against their consumers.  Foil this attack by using response
encryption to prevent the attacker from seeing the random sequence.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
 drivers/char/tpm/tpm2-cmd.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)
diff mbox series

Patch

diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index 001360d5956d..3ac438e8ffbb 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -289,29 +289,40 @@  int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 	int total = 0;
 	int retries = 5;
 	u8 *dest_ptr = dest;
+	struct tpm2_auth *auth;
 
 	if (!num_bytes || max > TPM_MAX_RNG_DATA)
 		return -EINVAL;
 
-	err = tpm_buf_init(&buf, 0, 0);
+	err = tpm2_start_auth_session(chip, &auth);
 	if (err)
 		return err;
 
+	err = tpm_buf_init(&buf, 0, 0);
+	if (err) {
+		tpm2_end_auth_session(auth);
+		return err;
+	}
+
 	do {
-		tpm_buf_reset(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_RANDOM);
+		tpm_buf_reset(&buf, TPM2_ST_SESSIONS, TPM2_CC_GET_RANDOM);
+		tpm_buf_append_hmac_session_opt(&buf, auth, TPM2_SA_ENCRYPT
+						| TPM2_SA_CONTINUE_SESSION,
+						NULL, 0);
 		tpm_buf_append_u16(&buf, num_bytes);
+		tpm_buf_fill_hmac_session(&buf, auth);
 		err = tpm_transmit_cmd(chip, &buf,
 				       offsetof(struct tpm2_get_random_out,
 						buffer),
 				       "attempting get random");
+		err = tpm_buf_check_hmac_response(&buf, auth, err);
 		if (err) {
 			if (err > 0)
 				err = -EIO;
 			goto out;
 		}
 
-		out = (struct tpm2_get_random_out *)
-			&buf.data[TPM_HEADER_SIZE];
+		out = (struct tpm2_get_random_out *)tpm_buf_parameters(&buf);
 		recd = min_t(u32, be16_to_cpu(out->size), num_bytes);
 		if (tpm_buf_length(&buf) <
 		    TPM_HEADER_SIZE +
@@ -328,6 +339,8 @@  int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 	} while (retries-- && total < max);
 
 	tpm_buf_destroy(&buf);
+	tpm2_end_auth_session(auth);
+
 	return total ? total : -EIO;
 out:
 	tpm_buf_destroy(&buf);